Both small-molecule and polymer organic substances are used in optoelectronic devices. Semiconducting small molecules include polycyclic aromatic compounds such as pentacene, anthracene and rubrene. The aromaticity of these molecules means that the electrons across all adjacent parallel aligned p-orbitals are delocalized, forming the π-bonds. Charge transport among molecules, or conduction, is made possible by the overlap between π-orbitals in neighboring molecules.
There is a dependence of charge carrier diffusion length, mobility and hence material conductivity on the degree of long range order in the semiconducting material, with the highest mobility reserved for single crystals. The exciton diffusion length has been shown to increase with the crystallinity of the material. Time-of-flight experiments have determined the charge mobility in organic molecular crystals to be between 1 and 10 cm2 V−1 s−1 at 300 K. Charge mobility is much lower than 1 cm2 V−1 s−1 for disordered organic semiconductors, where band transport is not applicable and carrier transport takes place by hopping between localized states. In between the two extremes, polycrystalline organic semiconductor's charge transport is dominated by traps attributed to grain boundaries and other structural defects. For reference, the electron mobility in amorphous silicon is about 1 cm2 V−1 s−1, and about 103 cm2 V−1 s−1 in crystalline silicon.
Among organic semiconductors, rubrene single crystals show the highest carrier mobility. Mobility values as high as 18-40 cm2 V−1 s−1 have been measured in rubrene single-crystal field-effect transistors (FETs). On the other hand, rubrene thin-film FETs showed mobility of only about 1.2×10−4 cm2 V−1 s−1, using an as-deposited rubrene film that is a mixture of amorphous and crystalline phases with disk-like crystallites, 5 micrometers in diameter, mixed with amorphous rubrene.
Understanding the crystallinity in small-molecule organic semiconductors is also important in device-specific ways. For organic thin-film transistors, the molecules should be oriented so that the π-π stacking direction is parallel to the channel to increase responsivity. For vertically oriented devices such as organic photovoltaics, the π-π stacking should be along the junction direction to enhance diffusion length. Additionally for OPVs, light absorption is typically optimal when the molecular plane (when it aligns with the transition dipole moment) is perpendicular to the incident light and thus parallel to the oscillating electric field.
Thus the structural order of molecules in organic semiconductors influences both charge transport and light absorption. Although single crystals have the properties that lead to high device performance, scalability considerations such as roll-to-roll processing make it necessary to use the material in the thin-film form. In order to have the best of both worlds—the practicality of thin films and the high performance of single crystals—it is hoped that as-deposited amorphous films can be annealed to form polycrystalline films with large grains, or low density of grain boundaries.
Efforts to produce polycrystalline thin-film rubrene of the desired crystalline phase are not easily reproducible, however, and mechanisms in the crystallization procedure that enable complete, polycrystalline films with large and uniform grains to form are not well explained.
Thus, a scalable, highly reproducible method for producing polycrystalline thin films, especially methods that can be generalized to other organic small-molecule semiconductors, is highly desired.